1. Field of the Invention
The present invention relates to a charger for charging or discharging a recording medium for forming an image in electrostatic electrophotography. More particularly, the present invention relates to a charger for uniformly charging a surface of a recording medium by corona discharge in an image forming apparatus such as a copying machine, a printer, etc., and having a discharging electrode having a plurality of projections in the direction of a rotational axis of the recording medium. The present invention further relates to a charger of an electrostatic electrophotographic system in which a current distribution ratio of electric currents flowing through an electrode of the charger and a grid electrode between this electrode and a photosensitive body can be adjusted and set to a predetermined current distribution ratio.
2. Description of the Related Art
As is well known, an image forming apparatus using an electrostatic electrophotographic system is constructed by processing sections of charge, exposure, development, transfer, separation, cleaning and discharge. Specifically, in a process for recording an image, a charger uniformly charges a surface of an image carrier as a recording medium formed on a conductive supporting body composed of e.g., an aluminum drum with respect to a rotated photoconductive layer. An optical image of an original image is next exposed onto the charged surface of the image carrier through an optical exposure device so that an electrostatic latent image according to this optical image is recorded on this carrier surface. Subsequently, toner is electrostatically attached to the electrostatic latent image on this image carrier and is then developed so that a toner image is formed on the image carrier surface. The toner image on the image carrier is then transferred onto a transfer material by a transfer device and is fixed by a fixing heater. Residual transfer toner left on the image carrier surface is removed therefrom by a cleaner and is collected in a predetermined collecting section. Residual charges are removed by a discharger from the image carrier surface after the cleaning operation to perform the next image forming operation.
For example, the recording medium as the image carrier is constructed by a photosensitive body in which an organic photo conductor (OPC) as the photoconductive layer is formed on the conductive drum. A corona discharger is generally used as a charger for providing charges for the surface of the recording medium in many cases.
In one corona discharger, a very thin conductive wire is covered with a conductive shield plate in a peripheral portion except for a shield portion opposite to the recording medium. A high voltage is applied to the wire so that corona discharge is caused to provide charges for charging by an electric current flooring through the recording medium. In another charger using corona discharge, a saw-toothed discharging electrode having many sharp projections arranged in line is utilized instead of the wire for corona discharge. A charging operation of this charger is performed by corona discharge from the sharp projections.
For example, the charger using the above saw-toothed discharging electrode (which is called a saw-toothed electrode in the following description) is proposed in specifications of the following patents.
(1) U.S. Pat. No. 4,591,713 (corresponding to Japanese Patent Application Laying Open (KOKAI) No. 60-158582) PA1 (2) U.S. Pat. No. 4,725,731 (corresponding to Japanese Patent Application Laying Open (KOKAI) No. 63-14176) PA1 (3) U.S. Pat. No. 4,725,732 (corresponding to Japanese Patent Application Laying Open (KOKAI) No. 63-15272) PA1 (4) U.S. Pat. No. 4,792,680 (corresponding to Japanese Patent Application Laying Open (KOKAI) No. 63-180977)
In a typical charger, a surface of the photosensitive drum is uniformly charged by corona discharge in an axial direction of this drum. A charging condition is Slightly changed in accordance with various conditions of the corona discharge. When the charging condition is changed, charging irregularities on the surface of the photosensitive drum are caused so that the quality of an original image to be formed is influenced by the charging irregularities.
A method for increasing the total electric current flowing through saw-toothed electrodes is considered a simple method for reducing these charging irregularities, However, when the total electric current is increased, it is necessary to increase a voltage applied to the saw-toothed electrodes. A discharging electric current is increased when the voltage applied to the saw-toothed electrodes is increased. Therefore, the amount of ozone generated from a discharging portion is increased so that the surface of the photosensitive drum is influenced by this ozone, thereby reducing the quality of an original image.
When the amount of ozone is increased, the ozone is bonded to various gases and foreign materials in the air floating within an image forming apparatus so that nitrogen oxides (NO.sub.X), silicon oxides (SiO.sub.2), etc. are generated. These oxides are attached onto surfaces of the saw-toothed electrodes and the grid electrode so that discharging ability of the saw-toothed electrodes and ability for controlling a charging potential of the grid electrode are reduced.
Further, it is necessary to prevent leakage discharge from tip portions of the saw-toothed electrodes to other unnecessary portions by an increase in applied voltage by increasing the total electric current. To prevent this leakage discharge, it is necessary to provide excessive distances from discharging portions of the saw-toothed electrodes to a shield case. Therefore, the shield case becomes large-sized so that the charger becomes large-sized.
As is well known, a copying machine of an electrostatic electrophotographic system is constructed by unit processes of charge, exposure, development, transfer and fixing. Specifically, in a copying process, uniform charges are provided by a charger onto a surface of a photosensitive body. Reflected light of an original image is irradiated onto this photosensitive body surface through an optical system so that an electrostatic latent image is formed. This electrostatic latent image is developed by electrostatically attaching toner as developing powder to the electrostatic latent image so that a toner image is formed on the photosensitive body. The toner image is next transferred onto a piece of recording paper by static electricity of the photosensitive body as a transfer body and is thermally fixed onto the piece of recording paper as an image according to the original image.
The photosensitive body is constructed by using a material such as selenium having a high resistance and a high optical carrier generation rate. The charges on the photosensitive body are provided by corona discharge. A typical charger for generating the corona discharge is composed of a wire electrode charger and a saw-toothed electrode charger having a needle-shaped electrode.
In the wire electrode charger, a charging line is made of tungsten or stainless steel having a thickness of 25 to 90 .mu.m (micrometers) and is tensioned within a charger case. The charger case is opened onto a side of the metallic photosensitive body through an insulator. The charging line is connected to a power source and corona discharge is generated from this charging line.
In contrast to the wire electrode charger having the charging line, the saw-toothed electrode charger has a saw-toothed electrode having a sharp tip portion and corona discharge is caused from this tip portion. In these chargers, a grid electrode having a predetermined potential is normally arranged between the photosensitive body and the charger to prevent a charging electric current from being changed.
It is desirable that the charger transfer uniform charges onto a photosensitive drum surface by the corona discharge. The corona discharge, however, is influenced by an external environment such as atmospheric pressure, temperature, humidity, etc. Further, an electrode current is changed by any wear of a tip portion of the saw-toothed electrode so that no photosensitive drum is uniformly charged with ionic charges. Any change in electrode current causes a change in impedance between the saw-toothed electrode and the photosensitive drum.
A grid current and a case electric current respectively flow through the grid electrode and the charger case by the corona discharge from the saw-toothed electrode. A ratio of the grid current to the case electric current is an impedance ratio and is constant in a normal operating state. However, this impedance ratio is changed in accordance with the above external environment and operating states of the electrodes.
The saw-toothed electrode is separated by a constant distance from the grid electrode and is fixed with respect to the photosensitive drum. Accordingly, when the impedance ratio is changed, charges on the photosensitive drum is not uniform and lack of uniformity on charges can not be corrected in a conventional charger.